The invention relates to the field of particle filters particularly used in an exhaust line of an engine for the elimination of soot produced by the combustion of a diesel fuel in an internal combustion engine.
The filtration structures for the soot contained in internal combustion engine exhaust gases are well known in the prior art. These structures most frequently have a honeycomb structure, one of the faces of the structure allowing the intake of the exhaust gases to be filtered and the other face the discharge of the filtered exhaust gases. The structure comprises, between the intake and discharge faces, a set of adjacent ducts or channels with axes parallel with one another separated by porous filtration walls, said ducts being stopped at one or other of their ends in order to delimit the inlet chambers opening on the intake face and the outlet chambers opening on the discharge face. For a good seal, the peripheral portion of the structure is most frequently surrounded by a coating cement. The channels or ducts are alternately stopped in an order such that the exhaust gases, as they pass through the honeycomb body, are forced to pass through the side walls of the inlet channels to join the outlet channels. In this manner, the particles or soot are deposited and accumulate on the porous walls of the filtering body. Most frequently, the filtering bodies used in motor vehicle exhaust lines are made of porous ceramic material, for example of cordierite or of silicon carbide.
In a known manner, during its use, the particle filter is subjected to a succession of filtration phases (accumulation of soot) and of regeneration (elimination of soot). During the filtration phases, the soot particles emitted by the engine are retained and deposited inside the filter. During the regeneration phases, the soot particles are burned inside the filter, in order to restore thereto its filtration properties. The porous structure is then subjected to intense thermo-mechanical stresses, which may cause micro-cracks that are likely over time to cause a severe loss of the filtration capabilities of the unit, and even its complete deactivation. This phenomenon is particularly observed on large diameter monolithic filters. Specifically, in the operation of an exhaust line, it has been observed that the temperature gradient between the center and the periphery of such structures increases as the dimensions of the monolith increase.
To solve these problems and increase the service life of the filters, filtration structures have more recently been proposed combining several monolithic honeycomb blocks or elements. The elements are most frequently assembled together by bonding by means of an adhesive or a cement of a ceramic nature, called in the rest of the description a joint cement. Examples of such filtering structures are for example described in patent applications EP 816 065, EP 1 142 619, EP 1 455 923, WO 2004/090294 or else WO 2005/063462. In order to provide a better relaxation of the stresses in an assembled structure, it is known that the heat expansion coefficients of the various portions of the structure (filtration elements, coating cement, joint cement) must be substantially of the same order. Consequently, said portions are advantageously synthesized based on one and the same material, most frequently silicon carbide SiC or cordierite. This choice also makes it possible to even out the heat distribution during the regeneration of the filter.
To increase the filtration surface of said filter, at a constant filter volume, filtering elements whose shape and internal volume of the inlet and outlet channels are different have been proposed, for example in patent application WO 05/016491. In such structures, the wall elements succeed one another, in cross section and along a horizontal and/or vertical row of channels, to define a sinusoidal or wavy shape. The wall elements undulate typically by a half sine wave period over the width of a channel.
To improve the thermomechanical strength of the elements having such a wavy shape of the channels and consequently of the filter assembled from said elements having this particular channel shape, a honeycomb structure has been proposed in WO 05/063462 whose ratio R, characterizing two adjacent peripheral channels, between the average thickness “E” of the set of outer walls and the average thickness “e” of the set of inner walls of said channels, is greater than 1.2. It is indicated in this application that such a configuration makes it possible to reduce the risk of cracks that may appear particularly when local high amplitude stresses are generated during the regeneration phases, particularly due to high temperature gradients existing within the filter and the different types of materials used for the monolithic elements and the joint cement.
Although such an increased thickness of the walls indeed makes it possible to significantly improve the thermomechanical strength of the filters assembled from such elements, it nevertheless also causes a not insignificant weight increase of the filters, typically of the order of 5.5 to 7.5%.
This weight increase has the disadvantage of increasing the thermal inertia of the filter, which causes an excess consumption during the regeneration phases and, when the filter incorporates a catalytic component, impairs its catalytic efficiency due to the increased activation (or energizing) time of the catalyzer.